The present invention relates to direct current machines, and more specifically to homopolar machines.
Homopolar machines are operated by direct current (DC) and are simple in design principle. They have been under consideration and development for some years for use in ship propulsion applications because of their high efficiency, compact size, low weight, and reduced acoustic signature relative to all other motors, see U.S. Pat. No. 3,657,580 (1972).
As is well known in the art, a homopolar machine includes four major components: armature; stator; field coils; and flux return. The armature is connected to the machine's shaft and may also be referred to as the rotor. The armature typically includes a series of concentric, copper cylinders and is free to rotate in a direction at right angles to the magnetic field lines produced by the field coils. When a voltage is applied across the armature in the direction of the shaft, electric current flows parallel to the shaft. The current and magnetic field interaction (I×B) results in torque generation and rotation, thus producing a motor. In contrast, if the armature is driven externally for a generator application, the interaction of the armature rotating at right angles to the magnetic field lines generates a voltage and electric current.
In both the motor and generator scenarios, current flows along the armature and to the stationary stator via sliding electrical contacts referred to herein as current collectors or brushes, which may take various forms and be made of various materials. Such materials include but are not limited to flexible fibrous copper and flexible copper strips; common to most DC motors, rigid material made from graphite or silver-graphite may also be used.
The field coils are typically circumferentially continuous in geometry and aligned on the same central axis with respect to each other. A homopolar machine always cuts (or crosses as it rotates) magnetic flux lines of a magnetic field in the same direction due to the interacting armature and shaft iron being aligned on the same axis. This means that any point on the rotor always sees the same magnetic field as it rotates, and no differences in magnetic flux or multiple magnetic poles are encountered by conductive elements of the armature as it rotates. Hence the nomenclature “homopolar machine”.
The flux return is typically comprised of a highly magnetically permeable material such as iron or steel. The flux return is designed primarily to limit the undesirable stray magnetic field that radiates from the machine; therefore, it typically takes the form of a structural housing that surrounds the motor. In addition, the flux return may also be designed to help direct the magnetic field lines produced by the field coils into the armature interaction region to improve the machine's flux utilization.
Although conventional rotating machines are in wide use, brush wear has been a point of continuing concern. One disadvantage of homopolar machines is that they often tend to have lower reliability in comparison to standard DC motors. Specifically, homopolar machines use current collectors, i.e. brushes, to transfer current between each rotating armature turn and each stationary stator turn. The utility of homopolar machines is the heavily dependent upon current collectors that are potentially unreliable, a large source of efficiency loss, and maintenance problems. Presently, all current collectors need to maintain uniform contact pressure with the armature, and performance is measured in terms of current collector wear and current-carrying capability. Maintaining an ideal contact pressure is difficult because in a homopolar machine the current collectors are generally located in the magnetic field zone where the collectors are subject to bending and torque. Although some homopolar machines have been designed with mechanisms that help to maintain an ideal contact pressure, the size, weight, and cost penalties, that result are troublesome, along with the introduction of new sources of reliability problems. Thus, there is a need to reduce the wear and tear of brushes.